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Plate tectonics is the concept that the outer part of the earth is split up into a set of rigid, moving plates. These plates move because of slow convecting currents of hot rock inside the earth.

Plate Tectonics

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The Earth’s Structure

To understand plate tectonic theory, we need to know something about the structure of the earth.

Lithosphere - the outer rigid shell of the earth’s structure.

Mantle - layer or shell of the earth below the lithosphere and above the core.

Core - the central portion of the earth. The outer core is molten, while the inner core, even though just as hot, is a solid because of the increased pressure.

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Summary of Important Concepts

Earth is composed of layers. These layers have different chemical and physical properties. The main layers are the core, mantle, and two types of crust (oceanic crust and continental crust), and also the lithosphere and asthenosphere.

Elevations of different parts of the earth’s crust are controlled by isostatic equilibrium: the concept that the oceanic crust and the continental crust float buoyantly in the denser mantle beneath.

In plate tectonic theory, the Earth’s outer rigid surface -- the lithosphere -- is divided into moving segments called plates. These plates move away from one another, move toward each other, or slide side-by side past each other.

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Summary of Important Concepts, continued

The theory of plate tectonics explains many important features of Earth’s surface, such as:

- mid-ocean ridges and the earthquakes and volcanic activity there

- ocean trenches and the earthquake and volcanic activity there

One force that drives plate motion is heat-driven convection currents in the mantle. The heat is generated by the decay of radioactive elements within Earth.

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A Layered Earth - Density

Density is a key concept for understanding the structure of Earth.

Density measures the mass per unit volume of a substance.

Density = Mass

Volume

Density is commonly expressed as grams per cubic centimeter.

Water has a density of 1 g/cm³

Continental crust has a density of 2.7 g/cm³

Oceanic crust has a density of 2.9 g/cm³

The mantle has a density of 3.3 to 4.5 g/cm³

The fact that the mantle is denser than either type of crust is important: The less dense oceanic and continental crust float buoyantly in the mantle. This is the concept of isostatic equilibrium (more on this is explained below).

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Layered Earth

A cross section of Earth showing the internal layers. These layers can be described by their chemical and physical characteristics.

You should learn the difference between the core, mantle, and crusts (oceanic & continental), and the differences between the lithosphere and the asthenosphere

(explained below).

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Layered Earth - Chemical Properties

Note that Earth is density stratified, that is, each deeper layer is denser than the layer above.

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The layers listed above are distinguished based on chemical composition and density. Another important layered aspect of the Earth is layers distinguished based on their physical properties, in particular whether they are stiff and rigid, versus able to flow slowly. (Next slide.)

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Layered Earth - Physical Properties

Keep these differences between the chemical layers and the physical layers in mind in the following sections.

The outer ~100 kilometers of the earth includes both the crust and the upper part of the mantle. In this region the rock is cool, and therefore rigid (stiff and not easily deformed). This layer is called the lithosphere (crust plus uppermost mantle).

In contrast, below this layer for several hundred kilometers within the mantle is a layer in which the rock is so hot that it flows slowly. This layer is called the asthenosphere.

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Layered Earth - Isostatic Equilibrium

Think for a moment about the crust. Why doesn’t it sink into the mantle? Why do the continents stick up above the ocean surface? How are features such as mountains supported?

The concept of buoyancy is illustrated by a ship. The ship sinks until it displaces a volume of water equal to the weight of the ship and its contents.

Earth’s continental and oceanic crust are supported on the denser underlying mantle in a similar manner. Both types of crust “float” in the mantle. Instead of buoyancy, the term isostatic equilibrium describes the way the two types of crust are supported on the mantle.

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Isostatic Equilibrium - an example

This figure shows how the continental crust adjusts itself to maintain isostatic equilibrium.

A great weight, like the formation of a glacial ice cap, will cause the crust to slowly sag down into the mantle. After the ice melts, the crust will gradually rise back up.

Several places on earth are presently rising upward this way, because Ice Age ice caps have been melting.

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Layered Earth - Internal Heat

Where does the heat within Earth’s layers come from?

Heat from within Earth keeps the asthenosphere flowing. This allows the lithosphere to keep moving. The source of this heat is radioactive decay, given off when the nuclei of unstable forms of elements break apart.

This heat causes the rock of the mantle to flow very slowly by convection. Hotter areas of the mantle (shown here in RED) are less dense, and so rise upward, while cooler areas of the mantle (BLUE) are more dense, and sink downward.

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Wegener’s Theory of Continental Drift

Alfred Wegener gathered evidence in the early 1900’s that the continents on either side of the Atlantic Ocean were once joined to form a single large continent he called Pangea. His evidence was based on similarities of fossils, and large areas of rock, on either side of the Atlantic.

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Fossil Evidence

Fossil remains of the same organisms can be found on different continents.

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Lungfish

Remains of the modern-day lungfish are present in fossil records stretching back 300 million years.

These fish have lungs that allow them to survive dry periods by forming a “cocoon” in the mud and breathing quietly until the next rains.

There are species of lungfish in Africa, South America, and Australia. The distribution of lungfish reflects their origins in Gondwanaland before continental drift separated these continents.

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Alfred Wegener’s theory of continental drift was out of favor with the scientific community for decades. Eventually new technology provided evidence to support his idea. (Unfortunately this evidence did not come along until after his death -- sorry Alfred!!)��- Radiometric dating of rocks revealed that the oceanic crust is surprisingly young compared to the continents. Oceanic crust is not more than about 200 million years old anywhere.� �- Echo sounders revealed the shape of the Mid-Atlantic Ridge. � �- Seismographs revealed that volcanoes and earthquakes occur mostly in narrow belts. (See next slide.)

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Earthquakes show where plate boundaries are located, providing important evidence for movements of the earth’s plates. Notice on this figure that earthquakes occur in narrow zones on the earth. These areas correspond to the edges of tectonic plates. As the plates move against each other, they make earthquakes!

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Seafloor Spreading - A Key Idea

An idea proposed by Harry Hess and Robert Dietz in 1960 explained the development of the seafloor at the Mid-Atlantic Ridge. Rising convection currents in the mantle force the sea floor apart at the ridge, causing it to grow and spread: a process called sea floor spreading. As the sea floor spreads, the continents on either side drift apart. Thus the Mid-Atlantic Ridge conforms to the shape of the continents. The inset shows the center of the Mid-Atlantic Ridge.

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The Theory of Plate Tectonics

The ideas of continental drift and seafloor spreading were tied together in the theory of plate tectonics. Main points of the theory:

Earth’s outer layer is divided into moving lithospheric plates.

The plates move apart at mid-ocean ridges, in a process called sea floor spreading. Magma rising and solidifying at the ridge forms new oceanic crust. This crust spreads away from the ridge to make room for more magma to rise up and form more crust. This process causes many earthquakes at mid-ocean ridges.

The plates come together at oceanic trenches, where one plate dives down beneath another one and gets melted back into the mantle: a process is called subduction. This process causes many earthquakes and volcanoes near oceanic trenches.

These plates move because of convection in the underlying asthenosphere, and also the downward pull of the subducting plate.

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The Major Lithospheric Plates

The major lithospheric plates and their direction of relative movement are shown here. The boundaries between plates correspond to most of the earth’s earthquakes and volcanoes.

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Sea Floor Spreading

Rising convection currents of hot rock in the mantle cause new oceanic crust to form and spread apart at mid-ocean ridges.

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Subduction

The oceanic trench marks the location where one plate bends down and descends into the mantle beneath the other plate. Notice the earthquakes, and the formation of magma (and therefore volcanoes) resulting from the melting of the subducting plate.